In a wire electrical discharge machining apparatus, a wire as one electrode is running in an up-down direction and is arranged to be opposed to a workpiece as the other electrode that is controlled to move on a plane perpendicular to the wire running direction. A pulse discharge is caused in a machining gap between the wire and the workpiece (i.e., inter-electrode gap), and the workpiece is machined into a desired shape by utilizing heat energy generated due to the discharge.
In the wire electrical discharge machining apparatus, in a configuration for supplying power to the machining electrodes, the workpiece is directly connected to one electrode of a machining power supply and the running wire is connected to the other electrode of the machining power supply through a feeding point on which the wire is slidable. Generally, two feeding points are provided; one above and the other below the workpiece. In other words, there are two circuits in parallel on upper and lower sides of the workpiece as paths for a discharge current flowing in the wire.
The wire electrical discharge machining apparatus generally employs two machining power supplies: a sub discharge power supply for inducing spark discharge (pre-discharge) with small current and a main discharge power supply for supplying large current as a machining current after generation of the spark discharge to perform rough machining and finish machining.
In the wire electrical discharge machining apparatus, wire breakage sometimes occurs depending upon the machining conditions. If the discharge is concentrated at one point, the wire electrode is locally overheated, which results in wire breakage. Conventionally, various technologies have been proposed for preventing wire breakage by preventing the local overheating of the wire electrode (for example, see Patent Documents 1 to 3 and the like).
Specifically, a technology is disclosed in Patent Document 1 in which switching elements are provided on each current path from a main discharge power supply to upper and lower side feeding points for opening and closing the current paths individually so that a one-side feeding for supplying a main machining current from only one of the feeding points is performed, and an upper-side feeding only from the upper-side and a lower-side feeding only from the lower side are switched every predetermined number of continuously applied pulse voltages. With this technology, large current can be applied without overheating the wire electrode, enabling to prevent wire breakage due to the heat generation.
In Patent Document 2, a technology is disclosed in which switching elements are provided on each current path from a main discharge power supply to upper and lower side feeding points for opening and closing the current paths individually so that a one-side feeding for supplying a main machining current from only one of the feeding points is performed, and an upper-side feeding and a lower-side feeding are switched asynchronously. With this technology, occurrence of a concentrated discharge can be prevented, so that breakage of the wire electrode due to heating can be prevented.
In Patent Document 3, a technology is disclosed in which a device is provided for measuring a discharge position in an up-down direction in a machining gap based on a difference and a magnitude relation of current flowing from a sub discharge power supply to an upper-side feeding point and a lower-side feeding point, and switching elements are provided on each current path from a main discharge power supply to the upper-side feeding point and the lower-side feeding point for opening and closing the current paths individually. When spark discharge occurs on the upper end side in the machining gap, the upper-side feeding is performed, when spark discharge occurs on the lower end side in the machining gap, the lower-side feeding is performed, and when spark discharge occurs at the center of a workpiece in a thickness direction, an upper-and-lower-both-side feeding for supplying current from upper-and-lower-both sides simultaneously is performed. The local overheating of the wire electrode in the center in the up-down direction in the machining gap in which cooling effect tends to be insufficient can be prevented by switching the feeding system in accordance with the discharge position.
In a wire electrical discharge machining apparatus, as disclosed in Patent Document 2, machining liquid nozzles are generally provided on the wire running path between the upper and lower wire guides at positions that are close in the up-down direction with an opposing position to the workpiece therebetween, and a wire electrode is cooled and discharge machining swarf is removed by ejecting a high-pressure machining liquid into the machining gap from upward and downward.    Patent Document 1: Japanese Patent Application Laid-open No. S59-47123    Patent Document 2: Japanese Patent Application Laid-open No. H1-97525    Patent Document 3: Japanese Examined Patent Publication No. H6-61663